1. Field of the Invention
The present invention relates to an exposure apparatus and an exposure control method and more particularly to a slit scan type exposure apparatus equipped with an exposure control device for controlling an exposure amount and uniformity of luminous intensity to a sensitive substrate within a predetermined range wherein a mask and the sensitive substrate are scanned synchronously to expose a pattern of the mask on the photosensitive substrate an exposure method by use with the apparatus.
Also, the present invention relates to an exposure control device for controlling an exposure amount and uniformity of luminous intensity to a sensitive substrate within a predetermined range and an exposure control method for the same.
2. Related Background Art
In manufacturing semiconductors, liquid crystal display devices or thin film magnetic heads, etc. under photolithography technique, projection exposure apparatuses have been utilized in which the pattern of a photomask or a reticle (hereinafter called the reticle) is exposed via a projection optical system on a photosensitive substrate such as a wafer with photoresist applied thereto, a glass plate or the like. Recently, a chip pattern of a semiconductor, etc. tends to become large and in the projection exposure apparatus, it is required to expose a larger portion of the pattern of a reticle on the photosensitive substrate.
Also, as the pattern of a semiconductor, etc. becomes minute, improvement of resolution of the projection optical system is required. In order to improve the resolution of the projection optical system, the exposure field of the projection optical system needs to be enlarged, which but is difficult in respect to the design or manufacture. Especially, when using a reflective and refractive system as the projection optical system, there is a case that the shape of the exposure field with no aberration happens to be circular.
In order to deal with the problems of the tendency of enlargement of the pattern to be exposed and the limitation of the exposure field of the projection optical system, a method has been proposed in which a reticle and a photosensitive substrate are scanned synchronously with respect to, e.g., a rectangular, circular or hexagonal illumination area (hereinafter called the slit-shaped illumination area). That is, a so-called slit scan type projection exposure apparatus has been developed in which the pattern of a reticle larger than a slit-shaped illumination area on the reticle is exposed on a photosensitive substrate, as disclosed in U.S. Pat. No. 4,822,975.
Generally, in the projection exposure apparatus, the condition of a proper exposure amount and uniformity of luminous intensity with respect to a photosensitive material of the photosensitive substrate is determined. Therefore, the slit scan type projection exposure apparatus is also provided with an exposure amount control device which makes an exposure amount to the photosensitive substrate coincide with a proper exposure amount within a predetermined allowable range and maintains the uniformity of the luminous intensity of exposure lights to the photosensitive substrate within a predetermined level.
Also, recently, it is required to enhance the resolution of the pattern to be exposed on the photosensitive substrate. A method for enhancing the resolution is to use shortwave exposure lights. Among presently usable light sources, an excimer laser light source, a pulse oscillation type laser source (pulsed light source) such as a metallic vaporization laser light source or the like emits shortwave exposure lights. However, the exposure energies (light amounts) of pulsed lights emitted from the pulsed light source are fluctuated for the respective pulses within a predetermined range.
Consequently, the exposure amount control device is controlled such that after the end of exposure, an integrated exposure amount becomes a proper exposure amount within an allowable range. In the conventional exposure amount control device, when the average pulsed light amount of pulsed lights from the pulsed light source is &lt;p&gt; and the range of the fluctuations of the light amounts of the pulsed lights is .DELTA.p, the parameter .DELTA.p/&lt;p&gt; representing the fluctuations of the light amounts of the pulsed lights is deemed to become a normal distribution (actually random).
When the number N of pulsed lights emitted so as to be the proper exposure amount (to a certain area (pulsed light integrated area) on the photosensitive substrate which is scanned relatively with respect to an exposure area conjugate to the slit-shaped illumination area) is N, the exposure amount control device is controlled by using the fact that the fluctuations of the integrated exposure amount after the end of exposure becomes (.DELTA.p/&lt;p&gt;)/N.sup.1/2.
When performing exposures by use with the pulsed light source in the slit scan exposure method, how the light emission timing of the pulsed light source is set becomes a problem. For this matter, conventionally, when scanning the reticle and the photosensitive substrate synchronously, a light emission trigger signal is sent to the pulsed light source each time a stage on the side of the substrate is moved for a predetermined distance. That is, a length measurement unit (e.g., laser interferometer) which measures the moving amount of the stage on the side of the substrate for scanning the substrate is utilized. And, light emission is performed in synchronism with the output of the length measurement unit.
In the above conventional technique, although the fluctuations of the light amounts of the pulsed lights emitted from the pulsed light source is taken into consideration, the fluctuations of the light emission timing (the fluctuations of periods when the pulsed light source actually emits a pulsed light each time after a light emission trigger signal is sent to the pulsed light source) are not taken into consideration. However the inventors of the present invention found that the light emission timing of the pulsed light source affects the control accuracy of the exposure amount and the uniformity of luminous intensity.
Further, generally, in the length measurement unit (laser interferometer or the like), there are fluctuations of the time when after performing actual measurement, it outputs the result of the measurement. Therefore, in such a method for making the pulsed light source emit a pulsed light in synchronism with the output of the length measurement unit, the fluctuations of the timing of reading the measurement result is added to the light emission timing of the pulsed light source. Accordingly, it is not possible to maintain the control accuracy of the exposure amount and the uniformity of luminous intensity within the allowable range because of the influence of the fluctuations of the timing reading the measurement results and the light emission timing of the pulsed light source.